1. Field of the Invention
The present invention relates to a clock generation technique, and more particularly, to a technique of generating a plurality of clocks that are different in phase from each other.
2. Description of the Related Art
In recent years, there has spread an information recording device which irradiates an optical disc medium such as a compact disc (CD) or a digital versatile disc (DVD) with a laser beam that is emitted according to a recording pulse signal (recording clock) which is generated by a phase locked loop (PLL) circuit to record information. In the actual recording operation of the information recording device, in order to determine a recording position on an optical disc with a high precision, a cycle shorter than a cycle of the recording clock, for example, a clock of 1/16 or 1/32 of the cycle of the recording clock is used, and a higher recording quality is obtained as the cycle is shorter, that is, the resolution is higher.
There have been proposed various techniques for obtaining the clock that is high in resolution. For example, JP2006-294131 A discloses a technique of shifting the phase of the reference clock little by little to generate a multiphase clock. Although JP 2006-294131 A fails to suggest how to shift the phase of the reference clock, there is assumed that a plurality of differential circuits that are sequentially connected to each other are used. According to the technique, it is possible to obtain the resolution corresponding to the amount of delay of one differential circuit.
Also, there has been known a technique of using a multiphase clock consisting of outputs of the respective stages of a ring oscillator that is normally used in a voltage controlled oscillator (VCO) of a PLL circuit. For example, JP 2000-156629 A discloses a technique of obtaining a clock output having a resolution higher than the amount of delay of n (n is an integer of 2 or larger) differential circuits by the aid of a first ring oscillator and a second ring oscillator having the differential circuits connected in a ring configuration. In the technique, the output of an i-th (1≦i≦n) differential circuit of the second ring oscillator is phase-coupled with the output of an i-th differential circuit of the first ring oscillator, and the output of the i-th (1≦i≦n) differential circuit of the second ring oscillator is phase-coupled with the output of an (i+1)-th (1 when i=n) differential circuit of the first ring oscillator. With the above configuration, the output signal of the i-th differential circuit of the second ring oscillator has an intermediate phase between the phases of the outputs of the i-th differential circuit and the (i+1)-th differential circuit in the first ring oscillator. That is, the first ring oscillator and the second ring oscillator maintain oscillations in a state where their phases are temporally shifted by ½ of the amount of delay of the differential circuits that constitute the first ring oscillator and the second ring oscillator. As a result, a time interval of the phase state during the oscillation becomes as fine as ½ of the amount of delay of the differential circuits, thereby making it possible to increase the resolution of the oscillator circuit to twice of a limit that is determined by the amount of delay of the differential circuits.
There has been known the ring oscillator of VCO which is made up of differential circuits having differential input and output. FIG. 5 shows an example of the ring oscillator in which the four differential circuits DCELs (DCEL1 to DCEL4) are disposed. Each of the DCELs inverts the phase of a pair of input signals that are differential outputs of a previous DCEL, and outputs the inverted signals to a next DCEL with a delay of a given time (delay amount t). In the ring oscillator, the differential input and the differential output of each DCEL have the same cycle T (in the example shown, the cycle T is “8×t” because the number of DCELs is four).
When the above ring oscillator is designed in such a manner that the differential outputs of the respective DCELs are converted in level by the aid of a level converter to obtain the clock signal, the multiphase clock can be obtained.
FIG. 6 shows a clock that can be obtained by subjecting the differential outputs ON1 and OP1 of the DCEL1 and clocks that can be obtained by converting the differential outputs ON1 and OP1 in level. As shown in the figure, ON1 and OP1 are a pair of sine waves that are opposite in phase to each other, and have the same level and cross each other at a point of “T×½”. As a result of converting the levels of ON1 and OP1, a pair of clocks that are opposite in phase to each other can be obtained, and those clocks switch their levels at a point (crossing point) where ON1 and OP1 cross each other.
Likewise, when the levels of ON2 and OP2, ON3 and OP3, and ON4 and OP4 are converted, respectively, the clocks shown in FIG. 6 can be obtained from the differential outputs of the respective DCELs. Since the outputs of the respective DCELs are sequentially delayed by the delay amount t, the clocks that are obtained by converting the differential outputs of the respective DCELs in level are also sequentially shifted in phase by the delay amount t. The clocks of eight phases can be generated from the outputs of the respective DCELs of the ring oscillator shown in FIG. 5 in the above manner.
The multiphase clocks that are generated by converting the levels of the outputs of the respective DCELs of the ring oscillator which is constituted by the DCELs having the differential inputs and outputs as shown in FIG. 5 are sequentially shifted in phase by the delay amount t of the DCELs, thereby making it impossible to obtain the resolution that is equal to or higher than a resolution corresponding to the delay amount t of the DCELs. In order to obtain the resolution that is equal to or higher than the resolution corresponding to the delay amount t of the DCELs, it is necessary to generate a clock having a phase between the phases of clocks which are generated from the outputs of two adjacent DCELs.